This invention relates to a load rest for a pantograph jack and in particular to an improvement relating to a one piece load rest fabricated from a metal plate.
A portable jack is often stored in a vehicle to enable a driver to lift the vehicle to effect emergency repairs, for example, to change a tire. One popular type of jack for automobiles is the pantograph jack. Known pantograph jacks typically have four lifting arms hinged in a parallelogram at four joints. One joint is located on a base of the jack. Another joint is positioned at a load rest vertically above the base. Two other free floating joints are located on a horizontal diagonal at opposite corners of the parallelogram formed by the arms. When the free floating joints are drawn together in a horizontal plane the arms extend vertically to lift the load support with respect to the base and vice versa. The relative position of the free floating joints is controlled by a drive screw or threaded shaft which links them together. It is important that a pantograph jack have stability when lifting a load.
The shape and dimensions of a load rest make an important contribution to jack stability and to cost of manufacture. Manufacturing considerations usually dictate that a load rest has about the same width as the jack's upper pantograph arms because it is linked over the upper arms by a bracket. Further, the width of the jack's components, including the load rest, pantograph arms and base are kept to a minimum to reduce material costs and weight. On the other hand, stability considerations dictate that a load rest must engage sufficiently with a vehicle to secure it against slippage under various conditions and resist rolling or yawing of the vehicle that might upset the jack. For practical use in automobiles, it should also facilitate quick and easy use by an unskilled operator with minimal instruction.
The typical prior art one piece load rest is only two material thicknesses (the load rest bracket legs) wider than the outside width of the upper pantograph arm. This width is sometimes too narrow to provide adequate vehicle support during extreme jacking conditions (i.e., a soft shoulder or an inclined surface). Current approaches in the art to provide better support are two or three piece designs which are joined by welding and provide the structural strength for increased vehicle contact area. However, these designs add cost and weight to the jack.
One example of a prior art load rest is found in U.S. Pat. No. 5,135,201 which discloses a two piece load rest in which a bracket connects to a lift cap with tabs that fit through slots therein. Other examples of prior art load rests for pantograph jacks and other similar jacks are disclosed in U.S. Pat. Nos. 4,194,725, 1,701,314 and 4,848,733; German patents 2,936,002 and 3,033,956 and Great Britain Patent 2,145,392.
It is an object of the present invention to provide a one piece construction for a load rest that may be formed from a metal plate to simplify construction and to reduce the cost of manufacture while permitting the rest to be wider than the pantograph arms.